1. Field of the Invention
This invention relates to an optical recording medium for recording and/or reproducing the information by laser beam radiation and, more particularly, to an optical recording medium advantageously employed as a tape-shaped recording medium.
2. Description of the Related Art
An optical recording medium for recording and/or reproducing the information by laser light radiation usually has a base plate on which is formed a recording layer formed of an optical recording material. For recording, the laser light is converged on this recording layer to form pits. For reproducing the recorded information, the difference in reflectance between the pits and lands between the pits is detected. It is possible with such optical recording medium to narrow the track width as compared with that with the magnetic recording medium, for example, to enable high density recording. For this reason, it is attracting attention as a recording medium for mass storage medium and investigations are proceeding briskly towards improving the quantity of the storable information.
As such optical recording medium, a disc-shaped recording medium, such as a digital audio disc (so-called compact disc) or an optical video disc (so-called laser disc), is in popular use.
If it is desired to increase the amount of the information recorded on one disc, it is necessary to increase the pit density on the disc or to increase the diameter of the disc itself to increase the area on which the pits can be formed.
However, if the pit density is excessively increased in the optical recording medium, signal crosstalks may be produced due to playback resolution limits proper to the playback optical system to render it impossible to detect the information satisfactorily. On the other hand, if the diameter of the disc itself is increased, the playback apparatus needs to be increased in size. In addition, the large-sized disc is liable to be damaged while being inconvenient for transport or storage. For these reasons, it is practically difficult to increase the quantity of information storage of the disc-shaped recording medium beyond the current level.
Recently, researches and development have been conducted of a tape-shaped optical recording medium, as shown for example in JP Paten KOKAI Publication No. 55-86782 (1980), as an optical recording medium having a larger storage volume than that of the disc-shaped recording medium. Such tape-shaped recording medium may be increased in length to maintain a pit-forming area markedly larger than that of the disc-shaped recording medium. Since the tape remains unchanged in outer size even if it is increased in length, a large storage quantity may be provided while maintaining its practical merits such as facility in transport or storability.
Meanwhile, for recording and/or reproducing such tape-shaped optical recording medium by a recording and/or reproducing apparatus, the recording medium is caused to travel with its recording layer kept in sliding contact with a drum as in the case of other tape-shaped recording media. Therefore, for stable recording or reproduction, it is necessary to take measures for reducing the friction between the surface of the medium and the drum and for preventing damages to the recording layer. These measures are particularly critical when employing a drum-shaped optical system which is rotated at an elevated speed for achieving a high information transfer speed.
For these reasons, a protective layer of a UV curable resin or a cross-linkable resin is provided on the tape-shaped optical recording medium in the conventional practice for diminishing the friction and wear otherwise caused to the tape-shaped optical recording medium. Such protective layer is effective to inhibit the friction and wear or to inhibit changes in pit shape as disclosed in JP Patent KOKAI Publication No. 58-94145 (1983) as well as to prevent debris produced during pit formation from being scattered to inhibit resulting defects.
However, the UV curable resins or the cross-linkable resins cannot be said to be excellent in optical transparency required of a protective film for the optical recording medium. The optical recording medium having its protective film formed of such resin suffers from a drawback that the laser light is deteriorated when being transmitted through the protective film so that satisfactory recording and/or reproducing characteristics can not be achieved. Besides, the frictional resistance and wear resistance of such recording medium are also inferior such that the running performance or durability of the tape-shaped recording medium cannot be improved satisfactorily.